puppet: Move clearsessions cron job to app_frontend_once.

While this is a different system than I'd written up in #8004, I think
this is a better solution to the general problem of cron jobs to run
on just one server.

Fixes #8004.

puppet/zulip/manifests/app_frontend_once.pp

@@ -41,4 +41,12 @@ class zulip::app_frontend_once {
     mode => 644,
     source => "puppet:///modules/zulip/cron.d/calculate-first-visible-message-id",
   }
+
+  file { "/etc/cron.d/clearsessions":
+    ensure => file,
+    owner  => "root",
+    group  => "root",
+    mode => 644,
+    source => "puppet:///modules/zulip/cron.d/clearsessions",
+  }
 }

puppet/zulip_ops/manifests/staging_app_frontend.pp

@@ -17,14 +17,4 @@ class zulip_ops::staging_app_frontend {
     target => '/etc/nginx/sites-available/zulip-staging',
     notify => Service["nginx"],
   }
-  file { "/etc/cron.d/clearsessions":
-    ensure => file,
-    owner  => "root",
-    group  => "root",
-    mode => 644,
-    source => "puppet:///modules/zulip_ops/cron.d/clearsessions",
-  }
-
-  # Staging has our Apple Push Notifications Service private key at
-  # /etc/ssl/django-private/apns-dev.pem
 }